I guess it really did seem too good to be true. Following the report yesterday that Nathan Fillion would be appearing as Nova in the post-credit scene in Guardians Of The Galaxy director James Gunn took to twitter to personally debunk the exciting rumor.
— James Gunn (@JamesGunn) June 21, 2014
So there you have it. Obviously fans will continue to rationalize that this is simply the director trying to un-spoiler a spoiler with simple misdirection, but at this point this rumor can be put to rest.
Guardians Of The Galaxy is currently set to release on Aug 1st 2014.